A large-scale transcontinental river system crossed West Antarctica during the Eocene

Extensive ice coverage largely prevents investigations of Antarctica’s unglaciated past. Knowledge about environmental and tectonic development before large-scale glaciation, however, is important for understanding the transition into the modern icehouse world. We report geochronological and sedimentological data from a drill core from the Amundsen Sea shelf, providing insights into tectonic and topographic conditions during the Eocene (~44 to 34 million years ago), shortly before major ice sheet buildup. Our findings reveal the Eocene as a transition period from >40 million years of relative tectonic quiescence toward reactivation of the West Antarctic Rift System, coinciding with incipient volcanism, rise of the Transantarctic Mountains, and renewed sedimentation under temperate climate conditions. The recovered sediments were deposited in a coastal-estuarine swamp environment at the outlet of a >1500-km-long transcontinental river system, draining from the rising Transantarctic Mountains into the Amundsen Sea. Much of West Antarctica hence lied above sea level, but low topographic relief combined with low elevation inhibited widespread ice sheet formation.

CT data processing was performed with the ZIB edition of the Amira software (version 2017.39,(114).CT-images show particles >1 mm in yellow while lignite fragments and roots are displayed in green.Grain-size distributions are scaled in phi values from -7 (left) to -1 (right), but need to be treated with care.Note that the number of clasts >1 mm below the hiatus (contained in segment 9R-1A) is very low.

The supplementary tables include:
-Table S1: The Science Team of Expedition PS104 -Table S2: Documentation and analytical information of individual samples analysed for this study.-Table S3: Analytical details of LA-ICP-MS U-Pb, Lu-Hf isotopes and trace element measurements of detrital zircons contained in the sandstone of drill core PS104_20-2.-Table S4: Analytical details of LA-ICP-MS U-Pb, Lu-Hf isotopes and trace element measurements of zircons contained in rhyolitic pebbles of drill core PS104_20-2.-Table S5: Analytical details of LA-ICP-MS U-Pb and trace element measurements of zircons contained in rhyolitic bedrock from the Jones Mountains (Thurston Island Block).-Table S6: Secondary standards measured during U-Pb analyses.
-Table S7: Analytical details of LA-ICP-MS U-Pb geochronology, Sm-Nd isotopes and trace elements measurements of detrital apatite contained in the sandstone of drill core PS104_20-2.-Table S8: Analytical details of LA-ICP-MS U-Pb geochronology and trace elements measurements of detrital rutile contained in the sandstone of drill core PS104_20-2.-Table S9: Standards measured during Hf and Nd isotopic analyses.
-Table S10: Analytical details of fission track analyses of detrital apatite contained in the sandstone of drill core PS104_20-2.
-Table S11: Results of XRD Analyses of rhyolitic bedrock from the Jones Mountains compared to rhyolitic pebbles contained in drill core PS104_20-2.-Table S12: Compositional analysis of the clay fraction (<2 µm) contained in the drill core PS104_20-2.-Table S13: Results of the biomarker analyses of the sandstone of drill core PS104_20-2.

Fig. S1 :
Fig. S1: Scans and grain-sized distribution from recovered core sections of site PS104_20-2.CT data processing was performed with the ZIB edition of the Amira software (version 2017.39,(114).CT-images show particles >1 mm in yellow while lignite fragments and roots are displayed in green.Grain-size distributions are scaled in phi values from -7 (left) to -1 (right), but need to be treated with care.Note that the number of clasts >1 mm below the hiatus (contained in segment 9R-1A) is very low.

Fig. S2 :
Fig. S2: Apatite crystals revealing naturally etched fission tracks and other crystal defects.Fission tracks and crystal defects are highlighted by red arrows and red dashed lines.Photomicrographs were taken of internal surfaces of apatite mounted in epoxy through an optical microscope at 1000x magnification with transmitted light.

Fig. S3 :
Fig. S3: Comparison of rhyolite bedrock from the Jones Mountains and rhyolite pebbles from the Polarstern Sandstone.a: Example of a rhyolite pebble contained in the Polarstern Sandstone; b: Thin section of the rhyolite pebble in cross-polarized light; c: Same thin section as (b) in plane-polarized light; d: Hand specimen of a rhyolitic bedrock from the Jones Mountains; e: Thin section of the rhyolitic bedrock in cross-polarized light; f: Same thin section as (e) in crosspolarized light.Note the alteration states of the feldspars (F).

Fig. S5 :
Fig. S5: Histogram and probability distribution curves of U-Pb ages.a: U-Pb ages of detrital zircon; b: U-Pb ages of detrital rutile; c: U-Pb ages of detrital apatite.Major magmatic events and orogenies are also indicated.FLIP = Ferrar Large Igneous Province.

Fig. S6 :
Fig. S6: Zircon U-Pb data from rhyolitic pebbles, compared to rhyolitic bedrock from the potential source area.Upper panels: Concordia plots of zircon U-Pb data for rhyolite pebbles contained in the middle to late Eocene Polarstern Sandstone.Lower panels: Bedrock volcanic and volcaniclastic rocks from the Jones Mountains.The red ellipse represents the concordia age.Note that ages displayed for samples R.3010.10 and R.3010.12 are weighted mean single-grain concordia ages.

Fig. S8 :
Fig. S8: Tera-Wasserburg diagram showing U-Pb data from apatite and zircon.Red symbols refer to apatite samples (9.9 mbsf); blue symbols refer to zircon samples (26.7 mbsf).Red bar at upper array intercept for Eocene apatite is the range of crystalline basement 207 Pbc/ 206 Pbc values for West Antarctica (47), which anchor the apatite age calculation.

Fig. S9 :
Fig. S9: Results of apatite U-Pb / FT double dating.The figure shows U-Pb ages vs AFT ages for all double-dated detrital apatite grains.Black line connects points of equal U-Pb and AFT age.U-Pb and AFT error bars are 2σ and 1σ, respectively.